چکیده انگلیسی

There are various difficulties involved with comparing the effects of short-lived and long-lived atmospheric species on climate. Global warming potentials (GWPs) can be computed for pulse emissions of short-lived species. However, if the focus is on the long-term effect of a pulse emission occurring today, GWPs do not factor in the fact that if a radiative forcing is applied for a short period, the climate system has time to relax back to equilibrium. The concept of global temperature change potential (GTP) at a time horizon for an emission pulse has been proposed to circumvent this problem. Here we show how GTPs can be used to compare black carbon (BC) and CO2 emissions and the methodology is illustrated with two concrete examples. In particular we discuss a trade-off situation where a decrease in BC emissions is associated with a fuel penalty and therefore an additional CO2 emission. A parameter—which depends on the BC radiative effects, the BC emission reduction and the additional CO2 emission—is defined and can be compared to a critical parameter to assess whether or not the BC emission reduction wins over the fuel penalty for various time horizons. We show how this concept can be generalised to compare the climate effects of carbon dioxide against a set of short-lived species and to account for differences in climate efficacy. Finally, the need for additional research is discussed in the light of current uncertainties.

مقدمه انگلیسی

There is a pressing need for devising a metric that allows comparing the climate effects of short- and long-lived atmospheric chemical species. Such a metric would allow one to develop more efficient climate mitigation policies as well as to achieve better trade-offs between air quality and climate policies. For instance, Hansen et al. (2000) proposed that in order to reduce the risk of dangerous climate change the emphasis on emission reduction could be put on methane, black carbon (BC) and ozone precursors over the next 50 years, although they too argue that a reduction in CO2 emissions is also needed. Streets and Aunan (2005) highlight the potential of BC emission reduction in the household sector in China. Although they raise the possibility of including BC emission reduction as a post-Kyoto option for China and other developing countries, they do not propose any climate metric or framework to deal with BC emission reduction in a post-Kyoto protocol. Jacobson, 2002 and Jacobson, 2005 suggested that, despite their lower fuel efficiency, gasoline cars were better for climate than BC-emitting diesel cars. However, Jacobson's analysis relied on radiative forcing (RF) and climate equilibrium calculations, which is artificial and possibly misleading because any policy or technology will only be implemented for a finite period of time.
Global warming potentials (GWPs) have been introduced to compare the cumulative radiative efficiency of different long-lived greenhouse gases over a time horizon (see Section 2.1 for a definition). GWPs are used to weigh the emissions of different long-lived greenhouse gases and a basket of them can be traded against each other under the Kyoto protocol. Although GWPs can technically be defined for short-lived species as well, their usage is not well established and there is little literature on GWPs for short-lived species. Recently Bond and Sun (2005) estimated that, despite its very short lifetime as compared with CO2, the 100-year GWP for BC is 680. Bond and Sun suggested that there would be a climate benefit to cut BC emissions for a range of super-emitters even with a fuel penalty of 10%. Forster et al. (2007b) estimated a direct GWP for BC of 510 in reasonable agreement with Bond and Sun (2005). Reddy and Boucher (2007) further investigated how the direct GWP for BC depends on the region of emission. They found that the 100-year GWP for BC ranges from 374 for BC emitted in Europe to 677 for BC emitted in Africa. The regional differences in BC GWP mainly reflect differences in the BC atmospheric lifetime, which themselves are mostly due to differences in the regional efficacy of wet deposition. Reddy and Boucher (2007) also pointed out that the snow-albedo effect of BC is associated with an indirect GWP that would present even larger regional differences. In particular, it was argued that the total (direct and indirect) GWP for European BC could be as large as 1600 for a time horizon of 100 years. This argues for BC emission reduction as part of a portfolio of climate mitigation policies. However, it should be kept in mind that (i) the RF and GWP by BC are still fairly uncertain and therefore climate policies should rely on a conservative estimate if a trade-off with CO2 is involved and (ii) GWPs do not factor in the fact that a RF concentrated at the beginning of a time period is less effective in inducing climate change at the end of the time period as compared with a RF that decays more slowly over time such as that of a CO2 pulse (see e.g. Fig. 1 in O’Neill (2000)). Rypdal et al. (2004) also acknowledged that GWPs are not well suited for short-lived species and suggested that, because of the regional nature of the forcings, aerosol emissions could be best regulated as part of regional climate agreements linked to a global climate agreement. There are also co-benefits to further regulate aerosols and other short-lived species because of their impact on air quality, human health and ecosystems.Given the recent progress in modelling the climate effects of BC and other short-lived species and the growing political interest to regulate these species, it is timely to assess how to compare emissions of CO2 and BC. It is the purpose of this paper to do so using the framework of the global temperature change potential (GTP) climate metric introduced by Shine et al., 2005 and Shine et al., 2007. Section 2 summarises and discusses the concepts of GWP and GTP. We then illustrate in Section 3 how the concept can be applied to concrete policy-relevant issues. Section 4 shows how the methodology developed in Section 3 can be generalised to account for several short-lives species and different climate efficacies.

نتیجه گیری انگلیسی

The approach presented above provides a framework to analyse the relative climate values of emission reductions of short-lived and long-lived species. The concept is illustrated in the context of BC and CO2 with both a win–win and a trade-off situation. We also showed how the approach can be generalised to handle multiple short-lived species and differences in climate efficacies. While we believe that this work is relevant to policy-making, it should be used with caution and the following caveats ought to be stressed:
1.
There are other uncertainties involved in the calculations presented here. In particular, the IRF for the climate response used here has been derived from an experiment featuring a step change in a homogeneous radiative forcing. We need to investigate the climate responses to pulse homogeneous and inhomogeneous radiative forcings.
2.
We have followed Shine et al. (2005) and expressed the climate effect in terms of the change in global surface temperature. However, the climate metric to be used depends on the policy question. In some instances, a climate metric based on a physical climate parameter may not be appropriate. A trade-off policy that accounts for both the air quality and climate benefit of emissions reductions may eventually require an analysis in some monetary unit.
3.
There might be other dimensions to the trade-off. For instance, some particulate diesel filters require ultra-low sulphur fuel. Manufacturing this fuel may also be associated with a fuel penalty (Beer et al., year not specified). It is therefore important to have a lifecycle approach in these cases. Lifecycle approaches will be critically needed to compare biofuels and fossil fuels.
4.
The choice of an appropriate time horizon to evaluate the benefit of a climate policy is difficult and to a large extent this depends on the policy question that needs to be answered. Obviously, the longer the time horizon, the more importance is given to CO2 as compared with BC. However, the longer the time horizon, the smaller the residual climate effect, especially if the trade-off is close to compensation. The picture should therefore not just be “black and white” and a negative climate effect for a very long-time horizon has to be balanced against a positive climate effect occurring on shorter timescales and the magnitude of the residual climate effect.
5.
The 100-year GWP by BC has been included in the X parameter above, rather than made implicit in the analysis, because it is still fairly uncertain. It is therefore essential that the uncertainties surrounding the GWP and climate efficacy of BC are reduced before firm decisions can be made in trade-off situations involving BC and CO2.